Electrophotographic processes that produce full color presentations are increasingly turning to the use of liquid toners. In a liquid toner, charge control agents are chemically added to a carrier which contains pigment/binder components. A subsequent chemical reaction between the charge control agents and the pigment/binder components results in a charging of the pigment/binder components. The liquid toner is, per se, electrically neutral, however once an electric field is applied, charged particles migrate towards opposite electrodes. Many prior art liquid toner systems have employed a liquid carrier-wetted paper to allow for the mobility of the toner particles and counter charges. (Images that are completely dry become neutral as there is no charge mobility--without the liquid carrier.)
Liquid carriers used with liquid toners have characteristically exhibited high volatility. Such a highly volatile carrier liquid is easily evaporated by heat, pressure and/or air flow, thereby assuring a dry media sheet upon completion of the printing process. One volatile carrier that is widely used is Isopar G, a product of the Exxon Corporation. Such volatile carriers are flammable and consequently are impacted by today's strict regulations regarding the handling of flammable materials. Systems employing volatile carriers can be found in the following U.S. Pat. Nos.: 4,947,215 to Chuang; 4,708,460 to Langdon et al; 4,897,691 to Dyer et al.; 4,842,972 to Tavernier et al; 4,733,272 to Howe et al; 4,731,636 to Howe et al.; 4,731,635 to Szlucha et al; 4,796,048 to Bean; 4,727,394 to Bov, Jr., et al.; 4,745,432 to Langdon; 3,839,032 to Smith et al.; and 4,325,627 to Swidler et al.
Certain liquid toner systems employ an adhesive transfer approach which uses a sacrificial transfer means requiring a replenishable transfer media (see U.S. Pat. Nos. 4,337,303 to Sahyun et al.; and 4,863,543; 4,891,677 and 4,931,334 to Shiozowa et al.) U.S. Pat. No. 4,684,238 to Till et al. employs an intermediate transfer process, however the liquid toner drying process employs an electrically biased metering roll juxtaposed to an intermediate belt. Such a drying process is mainly effective when using a high volatility carrier liquid.
Lower volatility carrier liquids have been employed with liquid toners. To remove liquid from such toner deposits, the prior art has suggested the use of reverse rollers and a polyurethane roller which acts as a "blotter" to absorb excess developer liquid from a developed latent electrostatic image on a photoconductive surface. Such a system is shown in U.S. Pat. No. 4,286,039 to Landa et al. Landa et al. employ a closed cell urethane foam material with open surface cells for picking up the carrier liquid. Such a system creates image defects and non-uniform drying due to the texture of the roller.
U.S. Pat. No. 4,974,027 to Landa et al. discloses a liquid toner system employing an intermediate transfer roller which incorporates an external squeegee/rigidizing roller and an intermediate transfer roller. Both rollers rely on some image wetness to permit electrophoretic charging to maintain a toned image's integrity as it passes through each roller. More particularly, the intermediate transfer roller described by Landa et al. accumulates the four color planes from the electrophotographic surface and only then transfers the color image to a media sheet.
In U.S. Pat. No. 5,204,722 to Thompson et al., a system is described for transfer of wet toner images to paper directly from a photoconductor surface. It has been found, however, that achieving complete transfer of an image from a photoconductor surface to any surface other than smooth paper is difficult.
Accordingly, it is an object of this invention to provide an improved color electrophotography system employing a relatively non-volatile liquid toner.
It is another object of this invention to provide a color electrophotography system wherein complete image transfer is assured between a toned photoconductive service and a media sheet.
It is yet another object of this invention to provide an image transfer mechanism for a liquid toner which does not rely upon electrophoretic charge movement during the transfer process.